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National Coil Coating Association About Coil Coating
A modern coil coating line represents an investment of tens of millions of dollars and is capable of line speeds up to 800 feet per minute with 72-inch coil widths and material thickness up to .135 inch.
Coil Coating Paint Line Process
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Coil coating is a continuous and highly automated process for coating metal before fabrication. In one continuous process, a coil of metal, up to 72 inches wide moving up to 800 feet per minute, is cleaned, treated and painted. The process begins with unwinding a coil of metal and cleaning both the top and bottom sides. From there, the metal is chemically treated, primed and painted. Once the metal is painted, the metal is rewound in coil form and prepared for shipping. The accumulator tower is a structure that allows material to accumulate, which makes continuous operation of the coil coating process possible.
National Coil Coating Association
Alkaline Cleaners, Mechanical Brushes and Fresh Water Rinses are used to prepare metal strip surface for the Pretreatment process by removing surface contaminants and mill oils applied to prevent corrosion during transit and storage.
Cleaning and Pretreatments
Successful cleaning of the strip depends upon optimizing control of dwell time, bath temperature and chemical concentration.
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The cleaning and pre-treating sections of the coil coating process focus on preparing the metal for painting. During the cleaning stage, dirt, debris and oils are removed from the metal strip. Alkaline cleaners, mechanical brushes and fresh water rinses are used to prepare the strip for treating. Successful cleaning of the strip depends upon optimizing control of dwell time, bath temperature and chemical concentration. From there, the metal enters the pre-treatment section and/or a chemical coater whereby chemicals are applied to facilitate paint adhesion and enhance corrosion resistance.
National Coil Coating Association
Pretreatments or Conversion Coatings are chemical treatments applied to the metal strip after cleaning and prior to painting. They are designed to improve the surface reactivity of the metal substrate, promote paint adhesion and provide corrosion resistance.
Cleaning and Pretreatments
Pretreatments 1. Improve Surface Reactivity
2. Promote Paint Adhesion 3. Provide Corrosion Resistance
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Pretreatments are chemical treatments applied to the metal strip after cleaning and prior to painting. Key learning points about the cleaning and pretreating process to remember: Improve surface reactivity, promote paint adhesion and provide for corrosion resistance.
National Coil Coating Association Primers
Primers are applied before the finish coat to aid in the paint systems adhesion to the substrate, add additional corrosion resistance properties through the use of corrosion resistant pigmentation and to improve the paint system’s flexibility.
Thick film primers can further enhance abrasion resistance (i.e. falling sand) and corrosion resistance properties for high performance applications that may have previously required the thickness of a Plastisol.
Thick film paint systems typically offer a lower pencil hardness and may also be prone to Edge Frilling on sheared edges and to Cracking/Crazing when cold formed.
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After the metal is cleaned and pretreated, it enters the prime coat station for the primer to be applied. Primers are used to aid in paint adhesion, provide additional corrosion protection and assist with paint flexibility. Most primers are applied at a dry film thickness range of .15 mils to .35 mils (or 5 to 8 microns). For some end use applications, a thick film primer might be specified, which can improve abrasion resistance and corrosion resistance properties.
National Coil Coating Association
Prime - Coater, Oven & Quench
Prime Ovens have multiple zones that are independently temperature controlled to achieve the desired Peak Metal Temperature (PMT). The strip is water Quenched after exiting the oven.
Prime Oven
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Once the primer is applied in the coater, the metal strip enters the oven to cure, or dry, the coating. Prime ovens have multiple zones with different temperatures to achieve the desired Peak Metal Temperature. Peak Metal Temperature, or PMT, is the maximum temperature achieved by the metal substrate during the curing process. After the metal strip leaves the oven, water is typically applied to quench, or quickly cool the metal upon exit from the ovens. Quenching can be achieved by using air or water.
National Coil Coating Association Finish Coats and Backers
Finish Coats or Top Coats are applied after the prime coat to provide: The desired physical appearance or aesthetics Weathering Characteristics – Chalk, fade and gloss retention Physical Properties – hardness and flexibility
Backers are applied after the prime coat to provide: Additional corrosion protection Finish coat transit abrasion protection A consistent bottom side appearance
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After the primer is applied and cured, the metal strip enters the finish coat station where a top coat is applied. Top coats provide color, corrosion resistance, durability, flexibility and any other required properties.
National Coil Coating Association
Finish - Coater, Oven & Quench
Finish Ovens have multiple zones that are independently temperature controlled to achieve the desired Peak Metal Temperature (PMT). The strip is water Quenched after exiting the oven.
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After the top coat is applied to the strip, the metal goes through the finish oven for curing. The finish ovens are similar to the prime ovens.
National Coil Coating Association Thermal Oxidation Process
Coil line ovens use a thermal oxidation process that destroys the Volatile Organic Compounds (VOCs) found in the coatings, making the coil coating process environmentally sound.
Oven Thermal Oxidizer
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Coil coating lines utilize a heat generated oxidation process for destroying volatile organic compounds. The VOCs are found in coil coatings and are removed and destroyed as the paint is cured through the ovens. This destruction process is one reason the coil coating process is environmentally sound.
National Coil Coating Association Application of Coatings
The Liquid Coating is picked up from the pan with the pickup roll. This roll typically rotates in a counterclockwise or reverse direction.
The Liquid Coating is then transferred from the pickup roll to the Applicator roll which rotates in a clockwise or forward direction.
Top Side Prime Coater Head
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Coil coating utilizes a roll applied coating application method with an “S” wrap coater design. The application process begins with the liquid coating getting picked up from the pan by a pick up roll. This roll typically rotates in a counterclockwise or reverse direction. The coating is then transferred to another roll called the applicator roll. The applicator roll rotates in a clockwise or forward direction. The “S” wrap coater design allows for primers and top coats to be applied to the top and back side of the metal strip in one continuous pass.
National Coil Coating Association Application of Coatings
The Liquid Coating is subsequently transferred from the Applicator roll to the metal strip.
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The coil coating application process provides for 100 percent transfer efficiency of the coating to the metal strip. With 100 percent transfer efficiency, the coil coating process is one of the most environmentally friendly methods for applying paint to metal because there is no overspray. Overspray of paint is an issue for most post painting application methods.
National Coil Coating Association
Most popular architectural paint finishes are 2-coat systems, resulting in a dry film thickness of about 1.0 mil. Courtesy of: Metal Roof Advisory Group, Ltd.
Top-Coat Cross-Sectional View
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On average, total coating thickness of a 2-coat paint system is approximately 1 mil or 25 microns. For comparison purposes, a one mil coating thickness is equivalent to the cellophane wrapper used in cigarette packs.
National Coil Coating Association
Attribute Test Method Color L,a,b Color Value Measurement
Dry Film Thickness Tooke – DJH - Electronic Gauges
Solvent Resistance/Cure M.E.K. Rub Test
Film Hardness Pencil Hardness Test
Gloss Gloss Meter
Flexibility T-Bend Test
Adhesion Impact Test
Corrosion Resistance Salt Spray Test
Water Resistance Cleveland Humidity Cabinet
Light Degradation Resistance QUV Test
Paint Attributes & Test Methods
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There are many test methods and procedures used in the coil coating industry to ensure quality products are produced. We will discuss some of the industry’s key tests.
National Coil Coating Association
Instrumental or Computed Color Difference: Numerical L,a,b color-scale values are used to standardize the perception of color and set the criteria for color control. These numbers provide a way to communicate color and form the basis to express color differences.
Color Control
L,a,b Color Solid
Visual or Perceived Color Difference: The magnitude and character of color differences are described by such terms as redder, bluer, lighter, darker, grayer or cleaner. These visual differences are used to control color.
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The majority of the coil coating industry uses a color reading system called the Hunter L,a,b. scale. The numerical L,a,b color-scale values are used to standardized the deviation of color and set the criteria for color control. These numbers provide a quantitative way to communicate color and form the basis to express color differences. For instance, the magnitude and character of color differences are described by such terms as redder, bluer, lighter, darker, grayer or cleaner.
National Coil Coating Association
Δ E = (Δ L )² + (Δ a )² + (Δ b )²
Color Differences
Simple Color Difference: Simple color difference is calculated by subtracting the “L, a, b” color-scale values of the standard from the sample.
Sample Value - Standard Value = Simple Color Difference (Δ Delta) If Δ L* is the sample is light If Δ L* is the sample is dark If Δ a* is the sample is redder (less green) If Δ a* is the sample is greener (less red) If Δ b* is the sample is yellow (less blue) If Δ b* is the sample is bluer (less yellow)
Total Color Difference: Total color difference or Delta E (Δ E) is calculated with the formula:
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The Hunter L,a,b color difference system works by calculating the sample value from the standard value and determining a color difference or Delta E. For instance, if the sample value is redder (or less green) then the color value in the sample will read plus some value on the “a” scale when compared to the standard. Likewise, if the sample value is reading greener (or less red) then the color value in the sample will read minus some value on the “a” scale when compared to the standard.
National Coil Coating Association Dry Film Thickness
A Tooke Gauge can be used to measure the Dry Film Thickness (DFT) of a coating system. A precision V-groove is cut into the dry film to allow for the thickness of each coating layer to be read using a 50X illuminated microscope.
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Another important measurement is Dry Film Thickness. As you recall from the discussion during the Coating Application process, the coating (or film) thickness is specified by the paint manufacturer. To measure the dry film thickness, we use a device called the Tooke Gauge which is a high powered microscope. A precision V groove is cut into the dry film with a tool to reveal the coating layers on the metal strip. The Tooke Gauge is then placed over the V groove so the quality inspector can read the film thickness of the layers of primer and top coat.
National Coil Coating Association Solvent Resistance/Cure
Solvent Resistance is an indicator of how well the paint is Cured. Liquid coatings undergo a chemical change during the curing process and become more Solvent Resistant. An M.E.K. Rub Test using a saturated cheesecloth is the most common way to determine if the coating has been properly cured.
A sample is rubbed at a 45° angle under moderate pressure. One forward + back motion = one double rub. Rubbing continues until the total double rubs specified by the paint manufacturer’s technical data sheet is met or a failure occurs.
Saturated Cheesecloth Gloved Hand
Painted Sample
Failure ~8 in. ~6 in.
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Besides ensuring the color is correct and the paint film thickness is correct, we must confirm that the paint is fully cured. To measure cure, we use a test called MEK Rub Test which measures solvent resistance. This test is an indicator of how well the paint cured. Liquid coatings undergo a chemical change during the curing process, becoming more solvent resistant. Using Methyl Ethyl Ketone (MEK) solvent, the quality inspector will saturate cheesecloth with the solvent. Then at a 45 degree angle under moderate pressure he will rub the coated product in a back and forth motion for a specified amount of rubs. If the paint remains intact, then the product is considered fully cured. If the paint is removed during this test, then the product is not cured and it fails. 1rub equates to 1 back and forth stroke.
National Coil Coating Association Film Hardness
Film Hardness is measured using a Beryl pencil. The pencil hardness is measured by pressing the pencil against the surface at a 45° angle and pushing the tip away from the operator with uniform downward and forward pressure to cut or scratch the film. The process is repeated down the hardness scale until a pencil is found that will not cut through the surface of the film. The hardness of the first pencil that is unable to cut the film indicates the hardness of the coating.
6B – 5B – 4B – 3B – 2B – B – HB – F – H – 2H – 3H – 4H – 5H – 6H (Softer) (Harder)
Hardness
of lead
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Pencil hardness is another test used by coil coaters. In this test, the hardness of the film is measured to ensure a prepainted product can be formed into a specified final product without the paint cracking or scratching. To perform this test method, quality inspectors will use a range of Beryl pencils with various hardness designations. The inspector will press a designated pencil against the painted sample at a 45 degree angle, pushing the tip away with a uniform downward and forward pressure to cut or scratch the film. The process is repeated, using a pencil with a different hardness value until a pencil is found that will not cut through the surface of the film. The hardness of the first pencil that is unable to cut the film indicates the hardness of the coating. A 6B pencil is the softest while a 6H pencil is the hardest.
National Coil Coating Association Gloss
Gloss is measured with a gloss meter. The gloss meter illuminates the surface of the coating to
confirm that the coating gloss is within a set range.
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Presentation Notes
To measure gloss, an inspector will use a device called a gloss meter. The gloss meter illuminates the surface of the coating to confirm that the coating gloss is within an established range. An example of a high gloss product is a reading of 80% at a 60 degree angle A reading of a low gloss product is 15% at a 60 degree angle.
National Coil Coating Association Flexibility
Flexibility is measured by forming a T-Bend.
3" wide samples are bent in the direction of the metal’s grain with the coating on the outside of the panel until the apex of the bend is as flat (i.e., no gap).
The bend radius is inspected for cracks at the specified T-bend radius using a jeweler’s loop. If no cracks exist, it is said to pass the No Crack (NC) test.
90° Bend 0T bend 1T bend 2T bend
NO GAP
NO GAP
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Presentation Notes
Understanding how a customer will manufacture their prepainted product is important to ensure the paint system specified will provide enough flexibility to make the part. The T-Bend test is an indicator of the paint’s flexibility. The operator will take a 3” wide sample of the painted product and bend the sample in the direction of the metal’s grain, coating side out, until the apex of the bend is flat. The bend radius is inspected for cracks. If no cracks exist, then the product passes the No Crack test.
National Coil Coating Association Flexibility
Pressure sensitive tape is applied to the bend radius, rubbed flat and removed with a rapid movement at an angle of 180° to the bend surface.
The tape is examined for any coating removal from the surface of the sample.
If no coating is removed, this satisfies a no-tape-off (NTO) requirement.
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In addition to the no crack test, the operator will also apply a pressure sensitive tape to the area, rub the tape flat, then remove it with a rapid movement. The tape is examined for any coating removal. If there is no removal of the paint, then this satisfies a no-tape-off requirement or NTO. An example of this test might be 2T, NTO, which means the paint passed a 2T bend with no-tape-off.
National Coil Coating Association Adhesion
Reverse Impact Testing
A round-ended weight is dropped onto the strip from a height that is 3 times the metal thickness.
The deformed surface is examined for cracks or breaks in the coating.
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The final test we will review is the paint adhesion test. This test confirms that the fully cured paint film has properly adhered to the metal. To perform this test, a round-ended weight is dropped onto the strip from a predetermined height. The deformed surface is examined for cracks or breaks in the coating.
National Coil Coating Association Adhesion
Reverse Impact Testing
Tape is applied to the deformed area, then removed. If any coating is removed from the surface of the strip, the sample has failed the test.
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Presentation Notes
Tape is then applied to the deformed area and removed. If any coating is removed from the surface of the strip, then the sample fails the adhesion test. No coating removal indicates good adhesion.
National Coil Coating Association
Attribute Test Method Color L,a,b Color Value Measurement
Dry Film Thickness Tooke – DJH - Electronic Gauges
Solvent Resistance/Cure M.E.K. Rub Test
Film Hardness Pencil Hardness Test
Gloss Gloss Meter
Flexibility T-Bend Test
Adhesion Impact Test
Corrosion Resistance Salt Spray Test
Water Resistance Cleveland Humidity Cabinet
Light Degradation Resistance QUV Test
Paint Attributes & Test Methods
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To produce a quality product that will perform in the field, it is important to remember that the coating must be on color, with the correct gloss level have the correct dry film thicknesses and be fully cured meet the customer’s specifications for paint hardness and flexibility and adhere to the metal coil There are other performance requirements that the customer might require such as weatherability. In those cases, additional testing may required.
National Coil Coating Association Industries That Benefit
Metal Doors Metal Roofing
Office Furniture Agricultural Equipment
Appliances
HVAC Beverage Cans
Automotive
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Prepainted metal is one of the most environmentally friendly, quality driven, and economical methods of applying paint to metal. Many industries benefit from the use of prepainted metal. In 2009, over three million tons of steel and aluminum were prepainted for the construction, automotive and consumer goods markets. More and more fabricators are realizing the benefits of using prepainted metal for their products.
National Coil Coating Association The Benefits of Prepaint
Energy & Environmental Benefits Substantial Energy Savings vs. Post Painting Trouble-Free Environmental Compliance Minimizes Waste and Emissions 100% Transfer Efficiency Closed loop process allows for the efficient capturing of
solvents and incineration heat recovery
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The highly automated and tightly controlled nature of the coil coating process is environmentally friendly. In addition, coil coating provides the highest-quality, most consistent coatings allowing for less paint to be used compared to conventional post-painting operations.
National Coil Coating Association The Benefits of Prepaint
Precision roll coating method that applies extremely controlled and uniform thicknesses of pretreatment, primer and top coat on the flat metal sheet, from edge to edge.
Corrosion Resistance.
Prepainted Metal
Post Painted Metal
AFTER 5 YEARS
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The tightly controlled nature of the coil coating process also improves the quality and performance of the painted product. From the technology used to clean and prep the metal, to the precise application method of applying the treatment, primer and top coat, the coil coating process yields better performing products compared to the post painting method.
National Coil Coating Association The Benefits of Prepaint
Specialized coatings can differentiate your products from competitors.
Antimicrobial
Non-Skid
Fingerprint Resistant Fingerprint Resistant
Reflective
Fingerprint Resistant
Non-Skid
Antimicrobial
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Depending on the end use requirements, coatings used in the coil coating process can be formulated for specialty markets. Specialized coatings such as anti-fingerprint, non-skid, fingerprint resistant and reflective can be used in the coil coating process.
